The detection of xenobiotics which the potential to induce autoimmune reactions is currently hardly possible. The popliteal lymph node assay initially developed in mice may prove useful in this regard. Results obtained with phenytoin in Sprague Dawley rats are presented. Single doses of phenytoin were injected into one hind footpad and the popliteal lymph node was excised and weighed 3, 7 or 10 days after injection. Only the 5 mg dose level produced a significant increase in popliteal lymph node weight, structural and cellular changes in the lymph node indicating a humoral stimulation was noticed in the males at 2 mg and in the males and females at 5 mg. These results warrant further progress in the development of this assay.